The invention relates to an internal combustion engine with at least one cylinder, in which the combustion of a homogeneous air/fuel mixture compressed in the cylinder by a piston is initiated by a time-controlled external ignition.
Such engines are called Otto engines in the literature. They can for example be designed as carburettor Otto engines, injection Otto engines or gas Otto engines, the latter being powered by a fuel that is gaseous in its normal state. In Otto engines, a homogeneous air/fuel mixture (variation of the air/fuel ratio lambda over the combustion chamber of less than 10%) is ignited via an external ignition means, usually a spark plug. Above all in stationary gas engines with ever higher specific performance values it has been shown that the lives of the spark plugs are not of satisfactory length. Attempts have therefore been made to increase the lives by applying coatings of noble metals, for example platinum alloys. This has also proved successful in some cases, but overall the life values are still not yet satisfactory. The fact that the electrode spacing has to be adjusted after a specific period of operation in spark plugs is also disadvantageous. This requires the switching off of the internal combustion engine.
Furthermore, it is known to run engines in lean mode, i.e. with a air/fuel mixture ratio lambda which lies well above the stoichiometric air/fuel ratio of lambda=1. Typical lambda values of such lean engines with a homogeneous air/fuel mixture in the case of natural gas are of the order of 1.4 to 1.7. In the most favourable case, values of up to 1.8 are possible. To reduce emissions of pollutants, in particular the NOx levels in the exhaust gases, a higher lambda value, thus a leaner mixture, would be advantageous. Tests by the applicant and the pertinent literature (for example “Internal Combustion Engine Fundamentals”, John B. Heywood, McGraw Hill Book Company, 1988, pages 403 and 426) clearly show, however, that with a spark ignition via spark plugs lean mixtures with a lambda value of more than roughly 1.7 are not ignitable in an internal combustion engine (Otto engine) with a homogeneous air/fuel mixture.